Digital cameras are becoming considerably popular electronic products in the world because of their adoption into nearly one billion mobile phones per year. Continuous pressure has been applied in the last ten years to reduce cost, complexity and size while at the same time increase image quality. These conflicting goals are in constant tension, but have produced remarkable cameras that fit in the smallest and thinnest mobile phones. Some examples of these innovations include: Complementary metal-oxide-semiconductor (CMOS) image sensors with ten times more pixels in the same image area; higher-quality yet lower-cost polymer and glass optics, ten times faster image capture electronics; and integrated lens actuation to automatically focus on both close-up (macro) and distant (infinity) objects. In a typical phone camera, the total track length of the optical system is less than 7 millimeters and movement required to focus from macro to infinity is less than 0.5 millimeters.
Auto focus (AF) technology is used in nearly one half of all phone cameras and the most common solution enabling the AF technology is a voice coil motor (VCM). A VCM auto focus camera suspends the lens in front of the image sensor with a flexure guide assembly. The flexure guide assembly holds the lens at fixed position as long as the external forces on the lens are constant. The VCM is a magnet and coil that surrounds the lens and generates a force in proportion to input current. The VCM force bends the flexure guide in a predictable manner such that input current to the coil approximately corresponds to lens position. One example of a VCM focusing lens module is described in U.S. Pat. No. 7,590,342, entitled, “Method and structure for suppressing response time of lens focusing structure,” which is hereby incorporated by reference in its entirety.
Unfortunately, VCM focusing modules have limitations including high power to drive current in the coil, continuous current and power to hold a lens position, non-straight flexure movement that adds angular tilt between the lens and image plane, low strength to withstand drop testing, slow stepping movement and oscillations extend the focus time and delay the time to take a picture. In addition, lens angular tilt is a significant problem for cameras with a resolution greater than five mega pixels. For a five mega pixels camera, VCM lens tilt is typically 0.3 degrees which results in inconsistent focus over the full image. Slow stepping with oscillations is a particular problem for VCM because video capture requires continuous auto focus (CAF). The typical VCM settling time is 30 milliseconds and for CAF this slow and unstable motion results in continuous changes in the image focus and shifting of the image position.
Piezoelectric ultrasonic actuators (also referred to as piezo motors) are being commercialized that address the limitations of VCM modules. Piezo motors generate ultrasonic vibrations, with micrometer-scale amplitude, in a controlled manner. A piezo motor has a vibrating contact point that touches a moveable surface and can move this surface over a long distance with bi-directional control. The piezo motor's tiny and fast vibrations are controlled so as to add together when frictionally connected to a moveable surface. A critical requirement of piezo motors is an integrated preload force that creates the friction force at that contact point. This preload force must be created without generating significant friction outside of the contact point, otherwise no motion or unreliable and imprecise motion will result.
One example of a piezo motor is disclosed in U.S. patent application Ser. No. 12/228,943, entitled “Semi-resonant Driving Systems and Methods Thereof,” which is hereby incorporated by reference in its entirety, in which a driving system including a structure and a vibration system is disclosed. The structure has at least one point to frictional couple to and drive a movable element in one of at least two directions. The structure also has at least two bending modes which each have a different resonant frequency. The vibration system applies two or more vibration signals which are at a vibration frequency to each of the bending modes of the structure. At the vibration frequency one of the bending modes of the structure is vibrating substantially at resonance and the other of the bending modes of the structure is vibrating at partial resonance. The vibration system adjusts a phase shift between the two or more applied vibration signals to control which one of the at least two directions the moveable element is moved.
Another example of a piezo motor is disclosed in United States Patent Application Publication No. 2008/0297923, entitled “Piezoelectric Actuator and Lens Driving device,” which is hereby incorporated by reference in its entirety, in which a preload member applying an elastic force for elastically supporting the piezoelectric actuator against a lens barrel to keep the tip friction member and the friction member in contact with each other is disclosed.
Yet another example of a lens actuator module that integrates a piezo motor with friction contact driving force into a camera system is disclosed in United States Patent Application Publication No. 2009/0268318, entitled “Lens driving module,” which is hereby incorporated by reference in its entirety.
Still yet another example of a lens actuator module that integrates a piezo motor with friction contact driving force into a camera system is disclosed in U.S. Pat. No. 7,426,081, entitled “Lens Transfer Device,” which is hereby incorporated by reference in its entirety. This lens actuator module applies the preload force to the piezo motor contact point directly on a pin in the pin-bushing guide bearing. A requirement of this conventional example is the friction of the pin must be much less than the friction of the piezo motor. Since the preload force in this example equals the piezo motor preload force which also equals the pin-bushing guide reaction force, the coefficients of friction can be directly compared. The friction coefficient of the pin-bushing μpin must be significantly smaller than the friction coefficient at the piezo motor drive contact μcontact. However, the ratio μpin/μcontact must be minimized by using special low-friction materials, lubricants or rolling element bearings for the pin-bushing guide yet at the same time motor contact friction must be high. Since these two components are essentially co-located in the module, material selection and manufacturing challenges are significant. An exemplary performance metric for the lens actuator modules is the Safety Factor (SF). SF is the ratio Gmax/G0, where Gmax is the maximum lens weight that the lens actuator module can lift against gravity and G0 is the actual lens weight. When μpin and μcontact are nearly equal, the safety factor (SF) of the lens actuator module approaches 1. A safety factor greater than 4 is desirable because the lens actuator module will produce nearly the same speed irrespective of gravity orientation. Another related exemplary performance metric is the Speed Ratio defined as a ratio of speed with and speed against gravity, and is equal to the ratio (SF+1)/(SF−1). Thus, for example, when SF=4, the Speed Ratio is 5/3=1.66. Similarly, when the SF=1.5, the Speed Ratio=2.5/0.5=5.
Unfortunately, these existing systems and methods may still suffer from slow or low-precision focus over the full image resulting from lens angular tilt, in addition to oscillations and unstable motions during a continuous auto-focus mode of operation, and high sensitivity to gravity orientation due to a SF close to 1. Further, the existing systems may be impractical to manufacture at low cost in high volumes.